1. Field of the Invention
The present invention relates to an electric power steering apparatus which assists a force required for steering operation with the turning force of a motor, especially to a control of a driving current given to the motor when a steering wheel is returned.
2. Description of Related Art
An electric power steering apparatus has been developed which provides a driver a comfortable steering feeling by driving a motor for assisting steering force on the basis of a detected result of a steering torque exerted on a steering wheel and by assisting a force required for steering a vehicle with the turning force of the motor.
The power steering apparatus consists of a rack shaft projected in the right and left direction of a vehicle body, the both ends thereof are connected to the right and left front wheels through each tie rod respectively, and a pinion shaft engaged with the rack shaft at the middle portion of the latter, interlockingly connected with the steering wheel.
Vehicles, provided with a steering mechanism of rack-pinion system for carrying out steering operation by changing rotations of the pinion accompanying the rotational operation of the steering wheel into a movement of longitudinal direction of the rack shaft, are divided broadly into the following two categories according to the set position of the motor for assisting steering force. That is, the one wherein, the shaft of the above said pinion is further projected from the engaged position with the rack shaft, the aforesaid motor being provided at the projected portion through an appropriate speed reducer in order to transfer the turning force, and the other wherein, an auxiliary pinion engaged with the aforesaid rack shaft is provided at a position in the axial direction different from aforesaid engaged position of the pinion, the aforesaid motor being provided at the aforesaid auxiliary pinion through an appropriate speed reducer in order to transfer the turning force, the former being named a one-pinion system, and the latter named a two-pinion system respectively according to the number of pinions engaged with the rack shaft.
In any power steering apparatus aforementioned, there has been a problem that return of the steering wheel after the steering operation tends to be disturbed by force of inertia of the motor and frictional resistance of the speed reducer, since the turning force of the motor for assisting steering force is transferred to the projected portion of the shaft of the pinion or the auxiliary pinion through the speed reducer. In order to solve this problem, an invention has been devised to perform return control of the steering wheel, shifting the characteristics in parallel according to the steering angle. (for example, Japanese Patent Application Laid-Open No. 61-89170, 1986)
In that invention, however, the characteristic of an indicator current for controlling the motor corresponds to the steering angle, thereby the assist characteristic during the time of steering operation and the return characteristic during the time of the returning change simultaneously in the whole region of the steering torque, whereby the assisting force is reduced when the steering angle is large, leading to a problem that the return characteristic and assist characteristic can not be set freely.
Another prior art device assists restoring force by the motor correspondingly to the vehicle speed. This reduces the quantity of indicator current to the motor corresponding to a decrease of the vehicle speed.
In the case where the indicator current is reduced to the motor corresponding to the decrease of the vehicle speed, however, the return control cannot be fully performed at low vehicle speed, thereby an appropriate neutral feeling of the steering wheel can not be obtained. That is to say, during the time of low vehicle speed, the friction of the wheels is large, and, large assisting force is required. In the case where the vehicle is to be housed in a garage while going backward in a low speed with a large steering angle, for example, a driver must perform a complicated operation such as looking at the steering wheel, while looking backward in order to return the steering wheel to the middle point, as the conventional returning control is weak in steering assisting force